1. Field of the Invention
The present invention relates to a methyl methacrylate resin composition and a molded article thereof. More specifically, it relates to a methyl methacrylate resin composition containing an ultraviolet absorber, and a molded article thereof.
2. Description of the Related Art
As a conventional method to improve the durability of a methyl methacrylate resin composition, a method including addition of an ultraviolet absorber to the composition is known. For instance, JETI, vol.46, No. 5, p.116-121 (1998) discloses use of a salicylate, cyano acrylate or benzotriazole compound as an ultraviolet absorber to be added to methyl methacrylate resins.
In the case where a methyl methacrylate resin blended with ultraviolet absorbers conventionally used is molded into an optical article, however, light transmitted through the optical article may be somewhat colored presumably due to the power absorbing viewable light of the ultraviolet absorbers used, hence, the transparency of the molded optical article may be insufficient. This problem becomes noticeable particularly when a transmitted optical path length of the molded optical article is long.
An object of the present invention is to provide a methyl methacrylate resin composition exhibiting superior durability without impairing the transparency inherent to a methyl methacrylate resin.
The present inventors made intensive have studied a methyl methacrylate resin composition exhibiting superior durability without impairing the transparency inherent to a methyl methacrylate resin, as a result, found that a methyl methacrylate resin composition containing a 2-(1-arylalkylidene)malonic acid ester exhibits superior durability without impairing the transparency inherent to a methyl methacrylate resin, and have completed the present invention.
That is, the present invention provides a methyl methacrylate resin composition comprising a methyl methacrylate resin and a 2-(1-arylalkylidene)malonic acid ester in an amount of about 0.0005 to about 0.1 parts by weight per 100 parts by weight of the methyl methacrylate resin.
The present invention will hereinafter be described in detail.
In the present invention, the methyl methacrylate resin is a polymer comprising not less than about 50% by weight of methyl methacrylate as a monomer forming the resin. An example of such a polymer is a polymer comprising not less than about 50% by weight of poly methyl methacrylate, which is essentially a monopolymer of methyl methacryate, or a copolymer comprising methyl methacrylate and not more than about 50% by weight of an unsaturated monomer, which is copolymerizable therewith.
Examples of unsaturated monomers which are copolymerizable with methyl methacrylate include methacrylic acid esters such as ethyl methacrylate, butyl methacrylate, cyclohexyl methacrylate, phenyl methacrylate, benzyl methacrylate, 2-ethylhexyl methacrylate, and 2-hydroxyethyl methacrylate; acrylic acid esters such as methyl acrylate, ethyl acrylate, butyl acrylate, cyclohexyl acrylate, phenyl acrylate, benzyl acrylate, 2-ethylhexyl acrylate, and 2-hydroxyethyl acrylate; unsaturated acids such as methacrylic acid and acrylic acid; styrene, xcex1-methylstyrene, acrylonitrile, methacrylonitrile, maleic acid anhydride, phenylmaleimide, and cyclohexylmaleimide. Two or more of these unsaturated monomers may be used in combination if necessary. The aforementioned copolymer may contain a glutaric acid anhydride unit or a glutarimide unit.
In terms of moldability, alkyl acrylates such as methyl acrylate are preferred as the unsaturated monomer, which is copolymerizable with methyl methacrylate. When such an alkyl acrylate is contained as the unsaturated monomer, the composition of the monomers used in the copolymer preferably comprises about 80 to about 99.5% by weight of methyl methacrylate and about 0.5 to about 20% by weight of the alkyl acrylate.
In terms of its lower water absorption property, styrene is preferred as the unsaturated monomer, which is copolymerizable with methyl methacrylate. When styrene is contained as the unsaturated monomer, the composition of the monomers used in the copolymer preferably comprises about 60 to about 90% by weight of methyl methacrylate and about 10 to about 40% by weight of styrene.
According to the present invention, a 2-(1-arylalkylidene)malonic acid ester is contained in the methyl methacrylate resin to provide a methyl methacrylate resin composition having superior transparency and durability. In terms of durability, an amount of the 2-(1-arylalkylidene)malonic acid ester is not less than about 0.0005 parts by weight, preferably not less than about 0.003 parts by weight, more preferably not less than about 0.005 parts by weight, per 100 parts by weight of the methyl methacrylate resin. In terms of transparency, the content of the 2-(1-arylalkylidene)malonic acid ester is not more than about 1 part by weight, preferably not more than about 0.05 parts by weight, more preferably not more than about 0.03 parts by weight, per 100 parts by weight of the methyl methacrylate resin.
Among 2-(1-arylalkylidene)malonic acid esters, those represented by the following general formula (1) are preferred in terms of durability; 
wherein X represents a hydrogen atom, an alkyl group having 1 to about 6 carbon atoms or an alkoxy group having 1 to about 6 carbon atoms, and R1 and R2 independently represent an alkyl group having 1 to about 6 carbon atoms.
In the formula (1), X is preferably a hydrogen atom, an alkyl group having 1 to about 4 carbon atoms or an alkoxy group having 1 to about 4 carbon atoms. The alkyl group represented by X or the alkyl group in the alkoxy group represented by X may be either in a straight-chain form or in a branched-chain form. Examples of such alkyl groups include a methyl group, ethyl group, n-propyl group, i-propyl group, n-butyl group, i-butyl group, s-butyl group, and t-butyl group. More preferably, X is a methoxy group.
Preferably, R1 and R2 in the formula (1) are each an alkyl group having 1 to about 4 carbon atoms. The alkyl group represented by R1 or R2 may be either in a straight-chain form or in a branched-chain form. Examples of such alkyl groups include a methyl group, ethyl group, n-propyl group, i-propyl group, n-butyl group, i-butyl group, s-butyl group, and t-butyl group. More preferably, R1 and R2 are each a methyl group.
The composition of the present invention may contain a hindered amine such as a compound having a 2,2,6,6-tetraalkylpiperidine skeleton for further improved durability. In this case, the content of such a hindered amine is usually not less than about 0.0001 parts by weight, preferably not less than about 0.001 parts by weight, more preferably not less than about 0.003 parts by weight, per 100 parts by weight of the methyl methacrylate resin. In terms of cost and transparency, an amount of the hindered amine is usually not more than about 0.1 parts by weight, preferably not more than about 0.05 parts by weight, more preferably not more than about 0.03 parts by weight. The content of the hindered amine per 100 parts by weight of the 2-(1-arylalkylidene)malonic acid ester is usually not more than about 100 parts by weight, preferably in the range of about 10 to about 80 parts by weight.
Examples of such hindered amines include a polycondensate of dimethyl succinate and 1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine, poly((6-(1,1,3,3-tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl)((2,2,6,6-tetramethyl-4-piperidyl)imino))hexamethylene((2,2,6,6-tetramethyl-4-piperidyl)imino, 2-(2,3-di-t-butyl-4-hydroxybenzyl)-2-n-bis(1,2,2,6,6-pentamethyl-4-piperidyl)butylmalonic acid, 2-(3,5-di-t-butyl-4-hydroxybenzyl)-2-n-bis(1,2,2,6,6-pentamethyl-4-piperidyl)butylmalonic acid, a condensate of N,Nxe2x80x2-bis(3-aminopropyl)ethylenediamine and 2,4-bis(N-butyl-N-(1,2,2,6,6-pentamethyl-4-piperidyl)amino)-6-chloro-1,3,5-triazine, bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate, bis(2,2,6,6-tetramethyl-4-piperidyl)succinate, and compounds represented by the following general formula (2); 
wherein Y represents a hydrogen, atom, an alkyl group having 1 to about 20 carbon atoms, a carboxyalkyl group having 2 to about 20 carbon atoms, an alkoxyalkyl group having 2 to about 25 carbon atoms, or an alkoxycarbonylalkyl group having 3 to about 25 carbon atoms. Two or more of these hindered amines may be used in combination if necessary. Among them, the compounds represented by the general formula (2) are preferable.
In the compounds of the above formula (2), each of the alkyl group, carboxyalkyl group, two alkyl groups in the alkoxyalkyl group (the alkyl group in the alkoxy group and the alkyl groups substituted with the alkoxy group), and two alkyl groups in the alkoxycarbonylalkyl group (the alkyl group in the alkoxy group and the alkyl groups substituted with the alkoxycarbonyl group) may be either in a straight-chain form or a branched-chain form. In the formula (2), Y is preferably a hydrogen atom or an alkoxycarbonylalkyl group having about 5 to about 24 carbon atoms. Particularly, a hydrogen atom or an alkoxycarbonylethyl group is more preferable as Y. Examples of such alkoxycarbonylethyl groups include dodecyloxycarbonylethyl group, tetradecyloxycarbonylethyl group, hexadecyloxycarbonylethyl group, and octadecyloxycarbonylethyl group.
The composition of the present invention may contain various additives if necessary. Examples of such additives include light diffusing agents or matting agents such as siloxane crosslinking resin particles, styrene crosslinking resin particles, acrylic crosslinking resin particles, glass particles, talc, calcium carbonate, and barium sulfate; antistatic agents such as sodium alkyl sulfonate, sodium alkylsulfate, stearic acid monoglyceride, and polyether esteramide; antioxidants such as hindered phenols; flame-retardants such as phosphoric acid esters; and lubricants. such as palmitic acid and stearyl alcohol. These additives may be used in combination of two or more of them.
As a method for preparing the composition of the present invention, there can be mentioned a method in which the methyl methacrylate resin, 2-(1-arylalkylidene)malonic acid ester, and, if necessary, a hindered amine or other additives are melt-kneaded with use of a single- or twin-screw extruder, or any one of various kneaders. Alternatively, it is possible to obtain the composition in a pellet form, a plate form or a like form by mixing a monomer forming the methyl methacrylate resin or a syrup containing a partial polymer of the monomer, the 2-(1-arylalkylidene)malonic acid ester, and, if necessary, a hindered amine or other additives together to allow bulk polymerization to proceed either batch-wise or continuously.
The composition of the present invention can be formed into various molded articles by molding processes such as extrusion molding, injection molding, and press molding. Examples of such molded articles include a signboard, a carport roof, an lighting cover, a front sheet for vending machines, a showcase, a toy, a lens, a prism, and a light guide. Among them, optical articles such as lens, prism, and light guide are preferable.
When the composition of the present invention is used for applications as optical articles in particular, a light transmittance of a molded article made from the composition with respect to light having a wavelength of 400 nm through a 30 cm-long optical path is usually not less than about 50%, preferably not less than about 70%. An average light transmittance of such a molded article with respect to light in the visible light range (the wavelength range of about 380 to about 780 nm) is usually not less than about 75%, preferably not less than about 80%.
The composition of the present invention can be suitably used for light guides, in particular, among optical articles. In recent years, a cold cathode fluorescent lamp exhibiting high efficiency and high luminance has been developed as a light source for a backlight of a liquid crystal display device, and with the development of such a cold cathode fluorescent lamp large-scale liquid display devices exhibiting increasing luminance are developed. Under such situations, light guides made from conventional methyl methacrylate resin compositions sometimes do not satisfy the requirement of transparency or durability. A light guide made from the composition of the present invention is excellent in both transparency and durability and hence can advantageously be used in a backlighting system incorporating a cold cathode fluorescent lamp having high luminance without causing deterioration of its own and of peripheral articles.
In molding the composition into a light guide, the composition may be molded into a sheet shape by extrusion molding using a T die or a roll unit, or may be molded into a sheet shape or a wedge shape by injection molding or press molding. Alternatively, the composition in a sheet shape may be prepared by cast polymerization.
A light guide made from the composition of the present invention is suitably used in the backlighting system of a liquid crystal display device with its screen having a width across corners measuring not less than about 14 inches, preferably not less than about 20 inches.